Aerodynamic bumpers are contoured to better match the shape of the underside of the hood and fender area. They curve under to replace flatter bumpers, which act as snowplows to the air.
Where OEMs diverge on aerodynamic bumper design is with air dams under the bumpers. OEMs have investigated a variety of options to optimize this area. The air dam tends to decrease the front clearance to the ground. This reduction in ground clearance causes the aerodynamic stagnation point to move. The location of the stagnation point affects the share of air that is routed under the vehicle, to the sides of the vehicle, into the grille, or over the top of the vehicle.
Due to lower drag.
Historically, add-on aerodynamic features for tractors incurred a higher initial cost. This is still the case for some of the optional devices.
Maintenance (Damage and Repair)
Some aerodynamic devices cover up other vehicle features that need periodic maintenance; devices can also be damaged during use.
Accessibility of Engine Compartment and Front of Tires
Some aero devices make it more be more difficult to reach engine.